Pakistan has begun building what independent analysts say is a powerful new reactor for producing plutonium, a move that, if verified, would signal a major expansion of the country's nuclear weapons capabilities and a potential new escalation in the region's arms race.

Satellite photos of Pakistan's Khushab nuclear site show what appears to be a partially completed heavy-water reactor capable of producing enough plutonium for 40 to 50 nuclear weapons a year, a 20-fold increase from Pakistan's current capabilities, according to a technical assessment by Washington-based nuclear experts.

The construction site is adjacent to Pakistan's only plutonium production reactor, a modest, 50-megawatt unit that began operating in 1998. By contrast, the dimensions of the new reactor suggest a capacity of 1,000 megawatts or more, according to the analysis by the Institute for Science and International Security. Pakistan is believed to have 30 to 50 uranium warheads, which tend to be heavier and more difficult than plutonium warheads to mount on missiles.

"South Asia may be heading for a nuclear arms race that could lead to arsenals growing into the hundreds of nuclear weapons, or at minimum, vastly expanded stockpiles of military fissile material," the institute's David Albright and Paul Brannan concluded in the technical assessment, a copy of which was provided to The Washington Post.

The assessment's key judgments were endorsed by two other independent nuclear experts who reviewed the commercially available satellite images, provided by Digital Globe, and supporting data. In Pakistan, officials would not confirm or deny the report, but a senior Pakistani official, speaking on condition of anonymity, acknowledged that a nuclear expansion was underway.

"Pakistan's nuclear program has matured. We're now consolidating the program with further expansions," the official said. The expanded program includes "some civilian nuclear power and some military components," he said.

The development raises fresh concerns about a decades-old rivalry between Pakistan and India. Both countries already possess dozens of nuclear warheads and a variety of missiles and other means for delivering them.

Pakistan, like India, has never signed the nuclear Non-Proliferation Treaty. One of its pioneering nuclear scientists, Abdul Qadeer Khan, who confessed two years ago to operating a network that supplied nuclear materials and know-how to Libya, Iran and North Korea.

The evidence of a possible escalation also comes as Congress prepares to debate a controversial nuclear cooperation agreement between the Bush administration and India. The agreement would grant India access to sensitive U.S. nuclear technology in return for placing its civilian nuclear reactors under tighter safeguards.

No such restrictions were placed on India's military nuclear facilities. India currently has an estimated 30 to 35 nuclear warheads based on a sophisticated plutonium design. Pakistan, which uses a simpler, uranium-based warhead design, has sought for years to modernize its arsenal, and a new heavy-water reactor could allow it to do so, weapons experts say.

"With plutonium bombs, Pakistan can fully join the nuclear club," said a Europe-based diplomat and nuclear expert, speaking on condition that he not be identified by name, after reviewing the satellite evidence. He concurred with the Institute for Science and International Security assessment but offered a somewhat lower estimate -- "up to tenfold" -- for the increase in Pakistan's plutonium production. A third, U.S.-based expert concurred fully with the institute's estimates.

Pakistan launched its nuclear program in the early 1970s and conducted its first successful nuclear test in 1998.